This invention relates to an electric power conversion system for conveying power between a direct current (d-c) power source and an electric load circuit, and, more particularly, it relates to a system of this type that includes a controllable converter and an improved electrical filter for partially isolating the converter from source transients and for attenuating harmonics developed by the system in operation.
Electric power conversion systems are used to condition the electric power supplied to motor load circuits from a d-c source of relatively constant voltage. If supplying d-c motors, such a system will include an electric power, "chopper" that is suitably controlled to vary the magnitude of load current and/or voltage is desired. Alternatively, in the case of alternating current (a-c) motors, the system will include an electric power "inverter" that is suitably controlled to vary the amplitude and frequency of load voltage as desired. In either case, electric power flows from the d-c source terminals to the load terminals of the controllable converter during "motoring" operation or in a reverse direction during "electrical braking."
Such a system is useful for propelling a rapid transit vehicle, in which case the source comprises a wayside conductor and the load comprises windings of at least one traction motor whose rotatable shaft is mechanically coupled through torque-increasing gearing to an axle-wheel set of the vehicle. The wayside conductor is typically emergized by a relatively low voltage d-c power generating plant located near the right of way along which the vehicle travels. In its motoring or propulsion mode of operation, the converter is so controlled that the d-c voltage applied to its source terminals is converted into adjustable voltage at its load teminals, and the traction motor(s) responds by producing torque to accelerate the vehicle or maintain its speed as desired.
In the alternative electrical braking or retarding mode of operation of the power conversion system, the converter is so controlled that each motor acts as a generator driven by the inertia of the vehicle and supplies electric power which flows in a reverse direction through the converter and appears as direct and unipolarity voltage at the source terminals. As this electrical energy is used or dissipated, the traction motor(s) responds by absorbing kinetic energy and slowing the vehicle. Electrical braking is achieved by a combination of dynamic braking and regenerative braking. Dynamic braking is effected by connecting a dynamic braking resistance between the d-c source terminals. This resistance receives current from the converter, converts the electrical energy to thermal energy, and dissipates the resulting heat. Regenerative braking, on the other hand, is effected by returning to the d-c power source power flowing in a reverse direction through the converter during braking operation. These two electrical braking modes can be combined in desired proportions, this mixing process being commonly referred to as "blending."
A power conversion system including a voltage source inverter for supplying a-c traction motors is disclosed in U.S. Pat. No. 3,890,551 - Plunkett, assigned to the assignee of the present invention and incorporated by reference in the present application. An important feature of the Plunkett power conversion system is its inclusion of ohmic resistance (shown at 28 in FIG. 1 of the Plunkett patent) that is inserted into the d-c link between the inverter and the d-c power source during electrical braking but is effectively removed from the d-c link during motoring. By inserting this series resistor during electrical braking, the magnitude of voltage at the d-c terminals of the inverter can increase above that of the source voltage. One of the advantages of thus raising the inverter voltage is to enable the traction motors to develop more magnetic flux for braking and to use less current than would otherwise be required for very high braking effort.
The power conversion system of the Plunkett patent also includes a low pass electrical filter of the conventional series inductance (L), shunt capacitance (C) type between the voltage raising resistor and the inverter for attenuating harmonics generated by operation of the inverter and for partially isolating the inverter from undesirable line transients. (As used herein, the term "harmonics" refers to various components of the composite current and voltage waveforms having frequencies that are multiples of the frequency of the fundamental component of such waveforms.) In addition, the shunt capacitance of the filter at the d-c terminals of the inverter provides the "stiff" voltage required for proper operation of a voltage source inverter.
The desired blending of dynamic and regenerative braking can be accomplished in various different ways that are well known to persons skilled in the art. See, for example, U.S. Pat. No. 4,093,900 - Plunkett. In the present state-of-the-art it is preferable to replace the parallel array of separate braking resistors and their respectively associated electromechanical switches, as shown in U. S. Pat. No. 4,093,900, with a single bank of resistance elements connected to the d-c link via an electric power chopper comprising a controllable solid-state electric valve that can be repetitively turned on and off in a pulse width modulation (PWM) mode to control the average magnitude of current in the resistor as desired. An example of this modern practice is disclosed in U.S. Pat. No. 4,761,600 - D'Atre et al, where the electric valve comprises a main thyristor or silicon controlled rectifier (SCR) and an auxiliary thyristor for commutating the main SCR from a conducting state (on) to a non-conducting or current blocking state (off). Alternatively, a solid-state gate turn-off device (GTO) could be substituted for the chopper shown in U.S. Pat. No. 4,761,600.